Engine ignition devices used for the longest time are electric spark ignition devices in which positive and negative electrodes are installed to maintain suitable intervals and high voltage electric discharge pulses are generated to ignite fuel in engines. Since such a spark ignition electrode device has a disadvantage in that its design makes it difficult to uniformly ignite to the utmost in an entire internal space of an engine, the spark ignition electrode device has a slightly lower fuel efficiency in the range of 16 to 18%. Since the 1990s, various types of laser ignition devices have been suggested to realize uniform discharge and to improve an ignition efficiency which is a problem of the spark ignition device. Although British and Japanese research teams suggested a laser ignition system and demonstrated that an engine efficiency in the range of 22 to 24%, which is improved by about 6% compared to an electric discharge method, can be realized, the laser ignition system still has low efficiency or lacks the specificity and economic feasibility needed to be actually applied to a vehicle engine, and thus commercialization has not yet been achieved. The main reason for this is that high cost pumping laser diodes are used, and thus actual application was almost impossible. As a specific example, for a conventional laser ignition device, research and development has been progressed in a method in which a laser medium to which mainly neodymium (Nd) is added is used, but a Nd-based laser medium, which is core laser technology, has a disadvantage in that an optimum pumping time thereof is in the range of 200 to 300 μs or less, and thus only an energy of a pumping laser diode pulse corresponding to a maximum of 300 μs per pulse can be used. In this case, the cost of a pumping laser diode, in which a usable pumping wavelength band is 800 nm and a required average output is 100 W or more, is four to five thousand dollars or more, and thus pumping laser diode is an optical component with a cost that is almost impossible to lower. Meanwhile, in 2012, a Japanese research team suggested a new laser ignition system in which laser pulse output light having a 1064 nm wavelength and generated by pulse-pumping the Nd-based laser medium with a laser diode having an 808 nm pumping wavelength is focused at high intensity on a very small diameter by a lens to generate light emission pulses to ignite fuel. However, the commercialization of such a system at low cost is estimated to be almost impossible even when a vertical-cavity surface emitting laser diode (VCSEL), which is a mass production type, is used.
Since a laser discharge system using such an Nd-based laser has an Nd ion having a short upper-state lifetime of 170 μs, a pumping pulse width of a pumping laser diode having a similar pulse width also has to be maintained in the range of about 200 to 300 μs, and even when a high cost 120 W pumping laser diode is used, only a pumping pulse energy in the range of 24 to 36 mJ per pulse can be input to the laser medium. As described above, the laser discharge system using an Nd laser has problems in that (1) a high cost 808 nm pumping laser diode has to be used, (2) since an upper-state lifetime of an Nd ion is 170 μs, an efficiency thereof is maximized when a pumping pulse width is about 300 μs at maximum, and thus a maximally usable pumping pulse energy is small and inefficient, (3) since the peak power of the pumping laser diode is high, a high current is used as an input power, and thus a driving circuit is complex, (4) accordingly, the entire system becomes expensive, and thus there is a problem in that actual application becomes almost impossible. (5) In addition, since there is a high possibility that combustion by-products, which are a problem when the system is actually applied and are generated during the combustion of fuel, adhere to a laser beam emission window, and an automatic cleaning function for a window inside a combustion chamber is not included, actual application is almost impossible. (6) Although there is a design in which an optical component having a high refractive index is applied for multi-focal combustion, when oscillation directions of output laser beams of multiple laser resonators are finely changed per pulse or beam divergence angles are changed, there is a high possibility that an output energy of the laser beams transmitted to a focal point in the combustion chamber through a focal lens is changed, and thus there is a possibility that incomplete ignition or ignition failure occurs.
Meanwhile, heat having a high temperature of about 2,000° C. is generated around an ignition point during an explosion stroke (expansion stroke) in an engine. Accordingly, there is a concern that the heat, which adversely affects the laser ignition device, can be transferred to the laser ignition device due to an emission spectrum, and thus there is a problem of insufficient preparation therefor in an ignition device of the conventional technology.